Strange ride last night. My predicted max heart rate is 181. Last night I did a 22 mile ride on my fixie. Even with some hills and pushing pretty hard I never got my rate over 170 ? I haven't ridden since Saturday. Yesterday afternoon was really hot here but I would have thought that would have made it easier to get my rate up... Any thoughts as to what could be going on. I felt OK... Not particularly strong, but OK. On a couple of the climbs I pushed well into the zone where lactic acid shut down my going any harder. Before the ride, driving over to the start, I noticed my rate was in the mid 50s... Normal for me just sitting very still.